A new finishing method of preparing durable antimicrobial nylon fabrics, developed recently, uses acid dyes as bridges to link antimicrobial agents such as quaternary ammonium salts to synthetic fabrics. The durability of antimicrobial functions is determined by the ionic interactions of the dyes with a quaternary ammonium salt, and by the add-on amount of the salt on the fabrics. Add-on amounts of the quaternary ammonium salt on fabrics are controlled by varying the concentrations of finishing baths. Curing time and curing temperature impact the durable antimicrobial functions of the finished fabric. When nylon fabric is treated with a solution containing 4% quaternary ammonium salt and then cured at 150℃ for 10 minutes, it maintains durable antimicrobial properties even after ten Launder-Ometer washings. The antimicrobial properties of the finished fabrics are also durable in conventional laundering. Furthermore, the finished fabrics possess good color fastness after ten Launder-Ometer washes.